1. Field of the Invention
This invention relates to data processing systems, and more particularly to the loading of firmware into a control stores of central subsystem.
2. Description of the Prior Art
Data processing systems include a central processing unit (CPU), a main memory, and a number of peripheral subsystems. The main memory stores instructions and operands. The instructions are transferred from main memory to the CPU where they are executed. The CPU performs operations specified by the instructions on the operands.
The CPU executes the instruction by decoding the order portion of the instruction and performing a number of predetermined microsteps. Early generations of CPU's performed these microsteps with hard wired logic. Later systems were made more versatile by using a read only memory for storing the microsteps, or firmware as it is now called.
Some of these present day CPU's store their firmware in random access memories (RAM's) thereby permitting a change in CPU personality by storing a different firmware set in the RAM, or as it is conventionally called a control store.
U.S. Pat. No. 4,396,981 entitled "Control Store Apparatus Having Dual Mode Operation Handling Mechanism" describes a data processing system with a writable control store.
Microprogramming and more particularly a writable control store are described in a book entitled "Microprogramming: Precepts and Practice", by Samir S. Husson, 1970, Prentice-Hall, Inc. Further U.S. Pat. No. 4,042,972 entitled "Microprogram Data Processing Technique and Apparatus" describes a computer system in which a writable control store is used in addition to the control store located internally in the central processor.